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Table of Contents

Train Describers, Berths and Train Descriptions
Variations in berths
Non-described descriptions
Mis-described trains
Non-interposing berths
ARS and the Train Describer
Terminus Stations
Blocking entry

Train Describers, Berths and Train Descriptions


A signaller needs a reliable method of identifying each train appearing on his panel. The word panel means a visual display of the area under the signaller's control and may be a wall display (hence the origin of its name) or, more recently, a screen display, also termed a VDU (Visual Display Unit). A typical wall panel has tracks painted on it, along which are lights that are lit by the passage of a train over track circuits (TCs) in the track.

Main line signals divide the tracks into blocks, each one allowed to contain only one train. In wall panels, a small four-character display box is inserted next to each signal and lights up with the train's ID when one is within the block. This display box is termed a berth, and the train's ID is referred to by a number of terms, the most usual being train description or simply description. Headcode is another term for this although its use is now outdated and somewhat colloquial.

Naturally, a panel with many tracks will contain many berths, and a train describer (abbreviated to TD) is an electronic device connected to the panel (one per panel) which controls what each berth displays. Train descriptions can be inserted by a signaller and, remotely, by others such as shunters. As the train moves along the line from signal to signal, the TD makes its description "step" from berth to berth (see below) to keep up with the passage of the train as shown by the lit TCs on the panel.

For this reason, berths are also referred to as "TD berths" since they are controlled by the train describer. You will also find "TD" used as an acronym for Train Description; the context in which it is used should make it clear whether the speaker is referring to a train describer or a description.

In SimSig, which is a representation of a common type of VDU panel, track circuits appear as red bars and berths as black rectangles to the rear of their corresponding signals, when facing along the direction of signalling. Platforms sometimes have more than one berth if more than one train can use the platform at a time, or if dividing/joining activities take place there.

A train description consists of four characters, either four digits, or a digit followed by a letter and two more digits. The first digit defines the class of the train and are broadly as follows:

0 - Light loco

1 - Express passenger or mail

2 - Local passenger

3 - Parcels or priority empty coaching stock

4 - 75 mph freight (usually container traffic)

5 - Empty coaching stock

6 - 60 mph freight

7 - 45 mph freight

8 - 35 mph freight

9 - Earlier than ~1990, unfitted freight. Later than 1990, International trains, Cross Country trains in the mid-2000s only, London-Scotland trains via Birmingham after 2013. Also local services on the East London Line after reopening.

The second digit or letter denotes the destination or route. These vary from area to area but a few usually denote inter-regional trains (former BR regions) or special types of trains:

E - London and North Eastern

I - European

L - Anglia

M - London Midland / North Western

O - Southern / European

S - Scotland

V - Western

X - Out of gauge trains, extra long trains, etc.

Z - Special trains

U - Underground (Tube) train, on North London Line

The third and fourth digits are progressive numbers which provide a semi-unique train identification. These numbers may wrap around (99->00) during the day if there are frequent services to the same destination or route.

There are two general conventions for numbering trains. One method is that all trains originating or terminating at a location will have one letter and the last two digits denote direction, even for one direction, odd for the other. For example, 1K34 and 1K36 are down London Liverpool Street to Southend Victoria trains, 1K35 and 1K37 are up Southend Victoria to London Liverpool Street trains.

The other method is to use the letter as the destination station or destination working timetable area. For example, 1G24 and 1G25 are both down trains from London Euston to Birmingham or Wolverhampton. Trains 1A24 and 1A25 are both up trains from Birmingham or Wolverhampton to London Euston. Similarly, 1A01 and 1A02 are trains running to Paddington (timetable area PA), 1B01 and 1B02 are trains running to the Cardiff area (timetable area PB).

A few places also had service codes - for example, all trains between A and B (say a branch line shuttle) would be 2B41.

It all depends on the region. Full instructions are provided with each simulation, but even then, there may be exceptions for individual trains. Looking at the timetable will resolve any queries as to the routing of the train.

Variations in berths

In SimSig, when there is no description in a berth, the berth is hidden, rather than displayed as an empty black rectangle. Note that many shunt signals seen in sidings, etc. have no berths associated with them as this is not a requirement, compared with main signals. Some berths (mainly seen on at the entry point to the controlled area, or at panel boundaries) are known as Approach (APPR) berths. These mirror the state of a berth, or set of berths, at the other signalbox. When several trains approach from the same direction, the identity of the train closest to your area is shown.

Non-described descriptions

If a train is in the area and it steps past a signal without a description in the 'from' berth, then the train will be given a non-described special description. For most regions, this is four stars (****) but the Southern region, being awkward, have a different one (*X**)!

Mis-described trains

Note that a train's description is really a label for your convenience, and is completely independent of the train's actual identity. If for any reason the train's description does not match the actual identity, the train is said to be mis-described. It will still follow the timetable assigned to it by its internal description, as seen on the Train List (F2).


Train description stepping is performed automatically by the train describer system. Where the description goes is based on the route set from the signal, and when it moves is decided by when the track circuit ahead of the signal becomes occupied. When the track circuit ahead of the signal becomes occupied, the description is removed from its current berth, and simultaneously placed in the berth of the next main signal the train will reach.

When a train enters the area from another signalbox's area of control, the train describer there will pass on the description to your area, often several minutes before the train actually comes under your control. Such berths are shown on the hollow track segments (but hidden when empty, of course).

When the train leaves your area, the description will clear out of the last berth. At some boxes, train descriptions leaving on a main (ie non-siding) line will go into a 'Last Sent' berth.


Interposing is the act of changing the state of a berth, be it filling the berth with a new description, removing a description, or replacing the existing description.

The most common need to change the description is when a service terminates in your area, and forms a new service. If you don't, the old description will continue to follow the train, causing confusion for yourself and the signaller at the next box (penalty points are given for sending a wrongly described train to a neighbouring signal box).

SimSig, being a piece of simulator software, manages its own train database, so if you mistype a train's TD you should be able to find the correct one by looking at the Train List (press F2) and finding the TD of the train whose location is closest to that red stripe you see on your screen! If you enter a TD for which there is no allocated timetable, SimSig will inform you so when you attempt to look it up; that should alert you to an error.

Note that there is no Undo function here. If you enter the wrong TD or inadvertently cancel it, you cannot undo this; you have to locate the correct one from the Train List or the timetable and enter it, so keep your wits about you when you interpose. If you doubt your memory, write down the old TD before changing it!

When a train divides or part of it becomes detached, each part requires its own TD and you will usually need to interpose it (see Dividing and Detaching trains). Conversely, when two trains merge to form a new one, it usually takes on a new TD which you must insert.

SimSig provides two methods for interposing a TD.

1) Select the train, then change its TD:

Right-click on the train's current berth on the panel. The context menu opens, with a choice of "Interpose", "Cancel" or "Show timetable". Left-click "Interpose" (or "Cancel" if you wish to cancel the TD). Pressing "I" or "C" respectively also achieve this. Type in the new TD. Although the berth only displays four characters, you can enter more than this and the full description will still be stepped. Although you won't be able to see all of it, it is good practice to enter the full description (e.g. 0K011). The berth is case-insensitive, i.e. typing "1a01" will still appear as "1A01".

When finished typing, click OK or press enter to insert the new TD.

2) Enter the new TD, then select the signal to which to attach it:

Press "I" (for Interpose) when the main window is active. The "Interpose/Cancel" window opens, displaying the message "Enter text to interpose, or blank to cancel". "Text" means the new TD. "Blank to cancel" really means "leave blank to cancel". It does not mean "type a space" (if you do, you will get just that - a gap in the red stripe!). To transfer the new td to your train, click "OK" (or press Enter), which closes the window. Next, left-click the signal adjacent to the train's current description and the description should change to the new TD. If no description is displayed for your train (anonymous red stripe), left-click the first running signal ahead of your train's red stripe and a berth should appear bearing the correct TD.

Note that the "Cancel" button displayed in the "Interpose/Cancel" window does not cancel the train's TD; it merely closes the window without taking any action.

Select whichever method you find more convenient.

SimSig automatically assigns a "non-described" TD, such as four asterisks (see the section on "Non-described descriptions", above) to any train that steps past a signal before the signaller has assigned a TD to it. You can remedy this by interposing its correct TD - the sooner the better!

Note that if you decide to change a train's working timetable, this will require more than simply changing its TD on your panel- the two are completely independent. To change the train's timetable, you must also change the Train List window, right-click the train and select Timetable Options > Run to another timetable from the context menu. See the 'Train List ' section for further information.

There is nothing to prevent you entering any four characters as a TD (for example certain codes entered in certain berths on certain Simulations will prevent trains entering, or force them to enter on a different line, e.g. Slow instead of Fast)


You may wish to cancel a description in a berth, perhaps because a track circuit failure caused a non-described description to appear or ARS has messed up again (!). Right-click on the description you wish to clear and select the 'Cancel' option that appears. The description will disappear. Note that if there was a train there and it subsequently moves past the signal, a non-described description will appear at the next signal.

Non-interposing berths

Not all berths permit you to interpose; these are generally beyond the boundary of your control area, and are controlled by another signalbox. They are there to give you advance warning of an approaching train.

ARS and the Train Describer

ARS uses the train describer to identify trains. If you change the description, then ARS assumes the train is to be routed according to the new train's timetable. ARS will also change the colour of a description. See the section on ARS for more details.

Terminus Stations

There are two methods of showing train descriptions at terminus stations, or in bay platforms.

The first method is to have a "last in" berth at the buffer end of the platform, and a "first out" berth at the signal end. Therefore, when a train steps into the platform and forms a new working, the old working will be displayed at the buffers, while the new working is displayed at the signal end. The old working will then disappear when the platform empties. There is a disadvantage to this method - if a second train enters the platform, the train descriptions may be the wrong way around.

The second method is to have a stack of berths, using a last-in, first-out method. The first train into the platform will step right the way along to the buffers. The train it forms will then be interposed on top of the old description. If a second train came along, then it would step to the second berth (either middle berth or signal end of the platform, depending how many berths there are in the platform). There is a disadvantage to this method also - the previous train identity is lost before the train physically arrives, the old identity being useful sometimes.

Blocking entry

In some simulations, it is possible to stop entry to the Sim or to divert entering trains to an alternative entry point. This ability is enabled by the Sim developer. An entry point is blocked for this purpose if the relevant berth contains one of the values BLOK, POSS, SHUT, or anything beginning *T.

Last edited by Edgemaster on 03/08/2018 at 16:53